Robert:
The best choice for the CS1 control is to find a suitable detector chip. Maxim has quite a number of parts to choose from. The MAX690 is a nice chip but much too expensive for this application. You can find other parts in their lineup that cost about 1/10 as much.

Your circuit with the 1K resistor and 10uF capacitor is not suitable. Maybe instead you could try this circuit while you wait for a good chip to arrive.

       +5V --+--------+
             |        |
             |        X
             X        X 10K
             X        X
             X        |
             |        +---------- CS1
             |      |/ 
             +------|  NPN
            |      |  
            ---       V
   Vz=4.7V  /       |
            ---       |
             |        |
             |        |
           -----    -----
            ---      ---
             -        -

For the power supply switch over I would use the type of circuit recommended in the data sheet. I.E. the one showing the PNP and NPN transistors. It is not the best to put a diode in series with the 5V supply to the RTC chip because the chip is then only running on 4.3 volts during I/Os and may not interface correctly via its bus lines. The PNP switch allows the VCC or the RTC chip to get right up to within 0.1 volts of the +5V rail.

I would think that a 0.1uF capacitor on the VCC line should be OK but 1 uF or 10 uF would work too as long as you choose one with very low leakage, as this capacitor ends up being a load on your battery (and a low quality capacitor could drain your battery faster than the RTC chip). If you do not mind the non-rechargable battery then the Li battery is a better choice. Using a NiCad rechargable with just a series resistor to charge it is a bad solution because there is no way to prevent over charging the NiCad in this configuration. For sure you will see plenty of second rate products that use a NiCad like that I would be hard pressed to recommend it.

MICHAEL KARAS